When the invigorating cold of winter gives way to spring, animals across the country emerge from hibernation and get ready to feed.
The first grizzly bear to appear in Yellowstone National Park was sighted on March 7, and thousands of other bears slowly followed.
“Bears in Minnesota come out of dens in late March and early April, but typically don’t leave the den area until mid-April,” said Andrew N. Tri, project manager for the Forest Wildlife and Populations Research Group at the Minnesota Department of Natural Resources news week.
“Males and solitary bears seem to go first. Females stay with cubs until the cubs are strong enough to climb. We don’t know the mechanism why they leave the burrows, but greening doesn’t usually happen until mid-May, so there’s a time when bears don’t have access to much food and continue to rely on their fat reserves,” Tri said.
The end of hibernation is triggered by the warming of the air in spring. Bears in the south tend to emerge earlier than those in the north as the temperature warms up first.
“The timing of emergence is very location dependent,” said Øivind Tøien, an assistant research professor at the University of Alaska’s Fairbanks Institute of Arctic Biology news week. “For example, bears in interior Alaska are known to come out later than coastal bears. Black bears in the southern US may have a very short breeding season. Lactating sows with young born during hibernation may also come out a little later or at least stay longer in the Denning area.”
Hibernation is a period of torpor and low metabolic activity that many mammals experience during the winter months. You experience a low body temperature, slower breathing, and a lower heart rate for several weeks.
“Hibernation in bears, and also in smaller mammals, is mainly about surviving a period of low food availability,” Tøien said. “The combination of accumulating fat stores when food is available and then suppressing metabolic rates during the winter allows them to survive the winter with those limited fat stores without burning protein and losing muscle mass.”
Before hibernation, the bears eat food, with grizzly bears gaining up to 3 pounds in body weight each day.
“In Minnesota, bears really only have an abundance of food for 3.5 months of the year. Very little food remains in the landscape after mid-October, so carry your head to burrows. If there was food all year round, I would imagine fewer bears would actually hibernate and only pregnant females would enter the dens to give birth,” Tri said.
It may be natural to assume that the bears are starving when they emerge from their dens, but in reality they are sluggish when they wake up.
“So when they emerge from their burrows, they’re not in a fully recovered, non-hibernating state,” Tøien said. “Because we’ve only studied bears in captivity here, it’s difficult to assess behavioral risk from our observations. My experience is that at this early stage after surfacing they are more likely to spend a lot of time resting, which is consistent with their metabolic state.”
The bears usually remain in their dens long after waking up, longer if they are females that have given birth to cubs over the winter.
“American black bears’ metabolism speeds up, they start stretching their muscles, their hearts start beating regularly, and they start breathing normally,” Tri said. “You don’t wake up very hungry. Females with cubs remain in the den area until the cubs are old and strong enough to climb trees.”
Therefore, the bears should not be more dangerous than usual when they surface.
“They are not inherently dangerous at all. It takes time before they want to eat again, and they start eating green vegetation (grass, clover, wetland plants) or other acorns that are lying on the ground and haven’t rotted over the winter. It will occasionally consume litter or bird seed that humans don’t properly secure in the spring,” Tri said.
As the effects of climate change spill over into the US, experts fear bears’ hibernation could be disrupted.
“Climate could alter the timing of emergence. In Alaska, the emergence may be related to the time when temperatures rise above freezing,” Tøien said. “Then meltwater can seep into the caves, making it uncomfortable and causing them to come out. This also appears to be the case in studies of European brown bears in Scandinavia. In theory, if the warming climate forces them out of their burrows earlier, but “green-up” doesn’t follow suit, they could run into energetic difficulties, but we don’t know if that will happen.”
However, bears can adapt to changes in their environment, so there may not be a major impact on any of the US species.
“We don’t really know what will happen to the bears as a result of climate change, but it’s important to remember that black bears are very adaptable,” Tri said. “They exist from the edge of the tundra to central Mexico in a variety of climates.”
“They do not overwinter throughout their range, but pregnant females hide (not always underground) to give birth and raise the young. Some have hypothesized that longer deportations would increase human-bear conflict, but I’m not sure if this applies to all areas of bear territory.”
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